In recent years, wireless communications systems have become common and the number of different types of communications systems has greatly increased. This is particularly true for commercial and military aircraft. In addition to communications systems, other types of wireless systems such as navigation and surveillance systems are common. Wireless systems require antennas to operate. Since the number of wireless systems has greatly increased there has been a corresponding increase in the number of antennas. Where a large number of antennas are required to be used in a small area, such as on a vehicle, problems with electromagnetic interference (EMI) can result.
New communications, navigation, and surveillance avionics systems are rapidly being added to aircraft. In commercial and general aviation aircraft, for example, high speed and large bandwidth communications systems that provide internet access and satellite television are being added. The trend in both military and commercial aircraft to add more communications and avionics systems is expected to continue, which in turn will likely cause an increase in the number of antennas. Given the limited external surface area of aircraft, as well as the aerodynamic considerations, a larger number of antennas necessarily means the antennas must be mounted closer together.
This increasing density of the antenna suite makes it more difficult to maintain inter-system electromagnetic compatibility. Antenna-to-antenna coupled EMI becomes an increasingly difficult issue. On some aircraft, for example, simultaneous operation of multiple avionics systems is currently not achievable because of EMI. This density of the antenna suite also makes it more difficult to successfully implement the traditional techniques for reducing EMI to more manageable levels.
Prior techniques for reducing or eliminating antenna-to-antenna coupled EMI include physically separating the antennas by a distance that ensures adequate space loss, installing radio frequency (RF) filters, frequency management, or installing interference blanking systems. As the density of the aircraft antenna suite continues to increase, selecting antenna locations that provide adequate space loss may not be possible. In addition, traditional RF filter solutions are typically not applicable for the in-band interference condition, while filter performance for out-of-band interference applications may not provide enough attenuation in the stop-band, or the transition band roll-off characteristic may not provide the required attenuation. Moreover, frequency management techniques limit the flexibility of system operations and may not be an acceptable alternative from the user's perspective, while interference blanking systems are inherently complex and do not provide for simultaneous multiple system operations. The blanking systems may also be cost prohibitive. As more communications and avionics systems are added to aircraft, maintaining inter-system compatibility using other traditional techniques for reducing EMI may become cost prohibitive. In addition, the density of the antenna suite may increase to the point where the traditional techniques for reducing EMI are simply not capable of preventing antenna-to-antenna coupled EMI.
While the problem of antenna-to-antenna coupled EMI is particularly acute on aircraft, this problem exists on other types of vehicles as well as stationary structures having dense antenna suites. Therefore it would be desirable to have an improved antenna system whereby antenna-to-antenna coupled EMI is reduced or eliminated.